Once upon a time there was a boy who was a great warrior of Light, but one day his Heart was lost. It was not lost to the Darkness, but hiding within the Light of another's Heart. So the boy slept and could not wake.
To protect him a good and powerful Enchantress cast a powerful spell on the castle that had once been the boy's home, transforming it into an impenetrable labyrinth. Any who tried to enter the castle risked losing not only themselves but also their memories. In the Heart of the Castle, she laid the sleeping boy in a room no one could find except for her. There he would be safe and sleep forever, or until his Heart could be returned.
The Enchantress left to seek her friend's lost Heart, but she too became lost to the world of darkness; a world from which very few have ever escaped. While she was gone, there was no one left to protect the Worlds from Darkness. And so the Darkness rose and swallowed up many Words, taking the form of monsters called Heartless.
As Words fell to Darkness, people too became fell to the Heartless. Their Hearts were lost to the Darkness and what was left of them faded into nothing, but sometimes what remained took form. These wraith creatures remembered who they were in life, and if their will was strong they looked as they had as humans, but they were Monsters all the same. Without Hearts they felt no love, no joy, no pity and no mercy. Their only goal was selfishness and the desire to return to what they once had been. For they remembered what it was to feel and longed to be Human again. These were the Nobody.
One of these Monsters was called Axel. His real name lost along with his Heart, he'd been given a new name by the Organization. A Group of Nobody willing to use whatever means necessary to achieve their ends. Axel was a cruel as any Nobody, a vicious unfeeling Monster. He even murdered his own comrades without a second thought, and cared for no one. For a Nobody could not care.
Then one day he met a boy, who changed everything.
Before that boy came to be, there was another boy called Sora. The Keyblade had chosen Sora, for although Sora did not know it long another Heart had to rest within him. That Heart was the Heart of a Warrior of Light who had once wielded the Keyblade himself. The Keyblade sensed that Heart and Sora's Light, and so it was Sora was chosen to save the Worlds from Darkness.
When Darkness claimed Sora's own world there was Princess there. Her Heart was pure Light, and so fled the Darkness taking refuse within the Light in Sora's Heart, just as the Heart of the Warrior had long before. The Princess fell into a deep sleep, and to save her Heart Sora had to free it from his own. So he used the Keyblade to cut open his own Heart, and became a Heartless himself.
However, the strength in Sora's Heart was so great that even in the Darkness he did not lose himself. He continued to fight, and when the Princess woke she used her Light to restore Sora to his human form.
Because Sora had lost his Heart to Darkness, even for a moment, a Nobody should have been formed. Except Sora's memories remained his him and he returned to human form with nothing missing, except that now the only Heart within him was his own.
The Heart of the Warrior had been freed when Sora freed the Princess's Heart. So it took form, in the shape of a boy who looked just like the Warrior had before his Heart had been lost.
The Organization found him, and thought he was a Nobody like them, Sora's Nobody. So they took his name, and gave him a new name: Roxas.
Roxas was not like the others who had their memories and felt nothing, Roxas could feel but remembered nothing of who he had been before he'd come to the Organization. He could no remember, for his Memories lay sleeping along with his true body, within the enchanted castle.
The Nobody were Form with Memories but no Heart. Roxas was Form with Heart but no Memories. However, because Roxas had a Heart, he could love. He came to love his friend Axel, and in doing so shared his Heart with the Monster. Roxas made Axel feel like he had a Heart, like there was more than just the nothingness he had known for so long.
Still, Axel remained a monster, like the rest of the Organization. When Roxas came to realize their evil he ran away. Axel begged him not to go. Axel wanted him around, because Roxas made Axel feel Human. Still, Axel cared only for himself. When the Organization told Axel to either bring Roxas back or kill him, he went after Roxas. When it was clear Roxas wouldn't return, he tried to destroy him.
Roxas went to Sora but found that Sora was asleep and could not wake unless Roxas returned to him. So his Heart once more rested within Sora's Heart and there it stayed.
Axel wanted Roxas back. He thought Roxas was a Nobody, so he planned to turn Sora into a Heartless so that Roxas would be reborn. He would rip Sora's very soul apart if that's what it took, kill him and condemn Roxas to their cursed existence, because Axel was a Nobody and could not truly care. All he knew was Roxas made him feel, and Axel would do anything just to see him again.
The Heart is strong, and no force more powerful than love. In the end Axel could not harm Sora, and gave his own life to save him. Because of Roxas a Nobody, a being he could not feel or care, gave his own life out of love.
So Axel faded, but this was not the end. If a Nobody is destroyed along with their Heartless they can be reborn. So Axel was reborn as his human self, a man named Lea.
Roxas remained in Sora, who still had much to do. He saved the Worlds a second time, but there were still those who needed to be saved. The Enchantress trapped in the Dark world, and the Warrior sleeping the castle.
In the End Sora saved the Enchantress, and went with her to the castle along with Lea. Roxas emerged from Sora's Heart and saw his own body lying there asleep. His Heart returned to its proper place.
Still the Warrior did not wake, and for a moment the group despaired. Then the Enchantress spoke. In her travels she had once seen a Princess fast asleep, and only true love's kiss could wake her.
"True love?" asked Lea. The man stepped forward and knelt down over the sleeping Warrior and placed a gentle kiss upon his lips. The Warrior's eyes fluttered open and he smiled.
"Axel?" he asked.
"No," he said, "My true name is Lea."
"Oh," said the man who looked so much like Roxas, except older, for he was twenty-five years old, the same age as Lea. "My name's Ventus."